Liquid crystals possess physical properties which are normally associated with both solids and liquids. Similar to fluids, the molecules in liquid crystals are free to diffuse about, however, a small degree of long range orientational and sometimes positional order is maintained causing the substance to be anisotropic as is typical of solids.
A vast array of organic and metal-containing substances exhibit liquid, crystallinity. A common feature of these molecules is either an elongated or flattened, somewhat inflexible molecular framework which is usually depicted as either cigar- or disk-shaped. The orientational and positional order in a liquid crystal phase is only partial, with the intermolecular forces striking a very delicate balance between attractive and repulsive forces. As a result, liquid crystals display an extraordinary sensitivity to external perturbations (e.g., temperature, pressure, electric and magnetic fields, shearing stress or foreign vapors).
The phenomenon of orientation, or anchoring, of liquid crystals by surfaces has been known nearly as long as have liquid crystals themselves. Anchoring of a liquid crystal by a surface fixes the mean orientation taken by the molecules with respect to the surface. This fixed direction is called the anchoring direction of the liquid crystal. A liquid population of mesogenic molecules can undergo a transition, between two or more anchoring directions, as a result of an external perturbation. Several anchoring transitions have been observed. These transitions involve a change in the orientation of the liquid crystal in the plane of the substrate which can be continuous or discontinuous. The transitions can be induced by a number of different perturbations, including the adsorption of foreign molecules. Such adsorption modifies the interface between the substrate and the liquid crystal, thereby inducing a "switching" between anchoring directions. See, Jerome, B; Shen, Y. R., Phys. Rev. E, 48:4556-4574 (1993) and Bechhoefer et al., Phase Transitions 33:227-36 (1991).
Past interest in the orientations assumed by liquid crystals near surfaces has been largely driven by their use in electrooptical devices such as flat-panel displays (FPDs). A goal of many studies has, therefore, been the development of methods for the fabrication of surfaces that uniformly orient liquid crystals over large areas. Future uses of liquid crystals in electrooptic devices, in contrast, will rely increasingly on liquid crystals with patterned orientations over small areas (Gibbons et al. Appl. Phys. Lett. 65:2542 (1994); Bos et al., J. Soc. Inf. Disp. 3-4: 195 (1995); Morris et al., Emmel, Proc. Soc. Photo-Opt. Instrum. Eng. 2650, 112 (1996); Mural et al., ibid, 1665:230 (1992); Patel et al., Opt. Lett. 16:532 (1991); Zhang et al., J. Am. Chem. Soc. 114:1506 (1992); W. P. Parker, Proc. Soc. Photo-Opt. Instrum. Eng. 2689:195 (1996)). For example, light can be diffracted or redirected by using patterned mesogenic layer structures that are tuned by application of a uniform electric field (W. P. Parker, Proc. Soc. Photo-Opt. Instrum. Eng. 2689:195 (1996)), and FPDs with wide viewing angles and broad gray scales can be fabricated by using pixels that are divided into subpixels, where each sub-pixel is defined by a different orientation of the liquid crystal (Schadt et al., Nature 381:212 (1996)). Methods capable of patterning mesogenic layers on curved surfaces are also required for the development of new types of tunable electrooptic mesogenic layer devices, including devices that combine the diffraction of light from the patterned mesogenic layer structure with the refraction of light at the curved surface (Resler et al., Opt. Lett. 21:689 (1996); S. M. Ebstein, ibid, p.1454; M. B. Stem, Microelectron. Eng. 32:369 (1996): Goto et al., Jpn. J. Appl. Phys. 31:1586 (1992); Magiera et al., Soc. PhotoOpt. Instrum. Eng., 2774:204(1996)).
Current procedures for the fabrication of patterned mesogenic layer structures use either spatially nonuniform electric fields from patterned electrodes or patterned "anchoring" surfaces. Fringing of electric fields from patterned electrodes prevents high-resolution patterning of mesogenic layers by this method (Gibbons et al. Appl. Phys. Lett. 65:2542 (1994); Williams et al., Proc. Soc. PhotoOpt. Instrum. Eng. 1168:352 (1989)).
Patterned anchoring surfaces have been prepared by using mechanical rubbing of spin-coated polymer films, photolithographic masking, and a second rubbing step performed in a direction orthogonal to the first (Patel et al., Opt. Lett. 16:532 (1991); W. P. Parker, Proc. Soc. Photo-Opt. Instrum. Eng. 2689:195 (1996); Chen et al., Appl. Phys. Lett. 67:2588 (1995)). This method of patterning mesogenic layers on surfaces is complex and suffers from the disadvantages of rubbing-based methods, such as the generation of dust and static charge. Recently developed photo-alignment techniques for orienting mesogenic layers provide promising alternatives (Gibbons et al. Appl. Phys. Lett. 65:2542 (1994); Schadt et al., Nature 381:212 (1996); Chen et al., Appl. Phys. Lett. 68:885 (1996); Gibbons et al., Nature 351:49 (1991); Gibbons et al., ibid 377:43 (1995); Shannon et al. 368:532 (1994); Ikeda et al., Science 268:1873 (1995); Schadt et al., Jpn. J. Appl. Phys. 34:3240.(1995)). However, because light-based methods generally require surfaces to be spin-coated by uniformly thin films of photopolymer, and because the orientations of mesogenic layers on photo-aligned surfaces are determined by the angle of incidence of the light used for alignment, these methods are not easily applied to the patterning of mesogenic layers on nonplanar surfaces.
The methods of the present invention permit fabrication of complex mesogenic layer structures in two simple processing steps. Surfaces can be patterned with regions of mesogenic layers that differ in shape and have sizes ranging from micrometers to centimeters. The mesogenic layers can also be patterned on nonplanar surfaces (mesogenic layers have been anchored within pores of alumina and vycor glass coated with surface-active reagents, Crawford. et al. Phys. Rev. E 53:3647 (1996), and references therein). The present invention differs from this past work in two principal ways. (i) Scale: Mesogenic layers have been anchored on curved surfaces with radii of curvature that are large compared with the thickness, of the mesogenic layer. The local state of the mesogenic layer is similar to that of mesogenic layers anchored on a planar surface and thus properties of the mesogenic layer are not dominated by elastic energies caused by curvature. Methodologies used for anchoring mesogenic layers on planar surfaces (for example, twisted nematic cells) can be translated to our curved surfaces; and (ii) Patterns: The methods of the invention allow the formation of patterned curved surfaces.
Self-assembled monolayers formed by the chemisorption of alkanethiols on gold are likely to now be the most intensively characterized synthetic organic monolayers prepared to date. See, Ulman, A., 1991, An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self Assembly (San Diego, Calif.: Academic Press); Dubois, L. H. et al., 1992, Annu. Rev. Phys. Chem., 43, 437. These monolayers form spontaneously during immersion of evaporated films of gold in solutions of alkanethiols as a result of chemisorption of sulfur on the (111) textured surface of the films. The molecules self-organize into a commensurate .sub.-- 3.times..sub.-- 3R30.degree. lattice on the surface of the Au(111). See, "Porter, M. D. 1987, J. Am. Chem. Soc., 109, 3559; Camillone III, N. et al., 1993, Chem. Phys., 98, 3503; Fenter, P. et al., 1994, Science, 266, 1216; 20; Chidsey, C. E. D. et al., 1990, Langmuir, 6, 682; Sun, F.; Mao, G.; Grainger, D. W. et al., 1994, Thin Solid Films, 242, 106. For monolayers formed from CH.sub.3 (CH.sub.2).sub.n SH, n&gt;9, at least, the aliphatic chains of the monolayers are extended in the all-trans conformation and tilted approximately 30.degree. from the normal of the surface. Because the spacing between sulfur groups on the .sub.-- 3.times.3R30.degree. lattice is, on average, 4.9 .ANG., whereas the van der Waals diameter of an aliphatic chains is, only .about.4 .ANG., the aliphatic chains within these SAMs tilt from the normal so as to come into van der Waals contact and thereby maximize their cohesive dispersive interactions. Studies of the lateral structure within monolayers using X-ray diffraction reveal the existence of domains of size .about.100 .ANG., where each domain has one of six different tilt directions relative to the Au(111) face. See, Fenter, P. et al., 1994, Science, 266, 1216.; Recent studies have shown the existence a c(4.times.2) superlattice, the cause of which remains unresolved.
Surfaces prepared by the chemisorption of organosulfur compounds on evaporated films of gold are not limited to the alkanethiols. Self-assembled monolayers formed from perfluorinated organosulfur compounds have also been reported. See, Lenk, T. J., et al. 1994, Langmuir, 10, 4610.; Drawhorn, R. A. et al., 1995, J. Phys. Chem., 99, 16511. These surfaces, too, can be highly ordered, although, interestingly, the origin of the order within the monolayer is largely intramolecular and contrasts, therefore, to monolayers formed from alkanethiols (where the order largely reflects the cohesive intermolecular dispersion force). Steric interactions between adjacent fluorine atoms of a perfluorinated chain cause the chain to twists itself into a rigid, helical conformation. That is, an isolated perfluoro chain is stiff, as compared to an aliphatic chain. Because perfluorinated chains have larger cross-sectional areas than alkanethiols, monolayers formed on gold from perfluorinated thiols are not tilted from the normal to the same degree as alkanethiols. See, Drawhorn, R. A. et al., 1995, J. Phys. Chem., 99, 16511. Estimates by IR studies place the tilt of the perfluorinated chains at 0.about.10.degree.. Because perfluorinated chains within SAMs on Au(111) are not tilted to the same degree as the alkanethiols, their surfaces are not expected to possess domains formed from regions of monolayer with different tilt directions (as occurs with monolayers formed from alkanethiols). Past reports do not describe the anchoring of liquid crystals on densely-packed monolayers formed from semifluorinated chains. Past investigations on fluorocarbon surfaces have focused on surfaces coated with films of fluorinated polymers such as poly-(tetrafluoroethylene) (Teflon.TM.) and poly-(vinylidene fluoride) (Tedlar.TM.), or fluorine containing surface reagents that pack loosely and host polar/charged groups. See, Cognard, J., 1982, Mol. Cryst. Liq. Cryst. Supp., 78, 1; Uchida, T. et al., 1992, Liquid Crystals Applications and Uses, edited by Bahadur, B. (New Jersey: World Scientific), p.2; Hoffman, C. L.; Tsao, M.-W; Rabolt, J. F.; Johnson, H. et al., unpublished results. Due to differences in the method of preparation (e.g., plasma polymerization of teflon vs. sliding contact of a teflon block) results reported in the past for the anchoring of liquid crystals on fluorinated surfaces are variable. In general, however, the fluorocarbon surface, which is a low energy surface, is reported to cause homeotropic anchoring. See, Uchida, T. et al., 1992, Liquid Crystals Applications and Uses, edited by Bahadur, B. (New Jersey: World Scientific), p.2.
The use of self-assembled monolayers formed from noon-fluorinated, semifluorinated and perfluorinated organosulfur compounds permits the preparation of well-defined fluorocarbon surfaces for the anchoring mesogenic layers. Mesogenic layers anchored onto these well-defined surfaces are sensitive to perturbations caused from a wide range of sources.
Numerous practical applications of the mesogenic layer's sensitivity to perturbation have been realized. For example, liquid crystals have been used as temperature sensors (U.S. Pat. No. 5,130,828, issued to Fergason on Jul. 14, 1992). Devices containing liquid crystal components have also been used as sensors for indicating the concentration of ethylene oxide in the workplace (U.S. Pat. No. 4,597,942, issued to Meathrel on Jul. 1, 1986). Meathral teaches a device which has an absorbent layer on top of a paper substrate. Neither Meathral not Fergason teach the switchable anchoring of liquid crystals on self-assembled monolayers. Additional information on the use of liquid crystals as vapor sensors is available in Poziomek et al., Mol. Cryst. Liq. Cryst., 27:175-185 (1973).
Liquid crystal devices which undergo anchoring transitions as a result of protein-ligand binding have been reported by Gupta, V. K., Abbott, N. L., Science 276:1533-1536 (1998). Switchable liquid crystals supported on self-assembled monolayers were used to detect the binding of avidin and goat-anti-biotin to a biotinylated self-assembled monolayer. Biotin exhibits a very specific, high affinity binding to both avidin (Kd.about.10.sup.-15) and anti-biotin (Kd.about.10.sup.-9). Gupta et al. teaches the binding of biotin to the self-assembled monolayer and the interaction of this ligand with proteins which are known to display specific and strong binding to biotin.
There is a recognized need in the chemical and pharmaceutical arts for devices having patterned liquid crystals as a component thereof. Further, there is a need for a facile method to produce such patterned liquid crystals. In addition, there is a long recognized need for both methods and devices to detect and characterize analytes in a simple, inexpensive and reliable manner. Even more desirable are systems that can detect the specific interaction of an analyte with another molecule. A device having a detection spatial resolution on approximately the micrometer scale would prove ideal for numerous applications including the synthesis and analysis of combinatorial libraries of compounds. If detecting molecular interactions could be accomplished without the need for prelabeling the analyte with, for example a radiolabel or a fluorescent moiety, this would represent a significant advancement. Quite surprisingly, the present invention provides such devices and methods.